continues (long article at source): Mental illnesses have been around for thousands of years. Evolutionary theory suggests that in order for them to be still here, there must be some kind of survival advantage to them. If they were wholly bad, it's argued, natural selection would have seen them off long ago. In some cases the advantage is clear. Anxiety, for example, can be a mental illness with severe symptoms and consequences, but it is also a trait that at a non-clinical level has survival advantages. In healthy proportions, it keeps us alert and on our toes when threats are sensed.

Interesting